We previously observed that mice with erythropoietin receptor restricted to erythroid tissue exhibit a disproportionate increase in fat mass and become obese, glucose intolerant and insulin resistant with decreased total activity and oxygen consumption. The obese phenotype appears greater in female than male mice. We found that treatment of wild type mice with erythropoietin increased hematocrit in male and female mice. However, a reduction in weight gain due to a decrease in fat mass and decrease in food intake was only evident in male mice on either regular chow diet or high fat diet, but not in female mice. To determine the role of female hormones on the loss of erythropoietin regulation of weight gain, female mice were ovariectomized and treated with erythropoietin. Erythropoietin treatment did not affect body weight in ovariectomized female mice on regular chow diet, but did reduce weight gain, fat mass and food intake in ovariectomized female mice on high fat diet, suggesting a protective effect to diet induced obesity that was also observed in male mice. The interference of estrogen on erythropoietin regulation of weight gain was demonstrated by estrogen pellet implantation in ovariectomized female mice. Treatment with estrogen provided protection against high fat diet obesity in ovariectomized female mice, and the combination of estrogen plus erythropoietin had no greater effect than estrogen alone. Estrogen was more effective than erythropoietin in decreasing fat mass in overiectomized female mice on high fat diet and the effects of estrogen and erythropoietin were not additive. This provided an explanation in part for the lack of response in body weight with erythropoietin treatment in wild type female mice and underscores the potential sex-differential response to erythropoietin in non-hematopoietic tissue. These studies in mouse models are consistent with our earlier report in full-heritage Southwestern Native Americans that showed the association between endogenous plasma erythropoietin level and weight change per year was negative in males and positive in females. In contrast, endogenous erythropoietin was negatively associated with hemoglobin in both males and females, indicating that the sex-differential erythropoietin regulation of weight change is related to the non-hematopoietic activity of erythropoietin, as suggested by erythropoietin activity in mice. Other tissues that contribute to erythropoietin metabolic response include the hypothalamus where erythropoietin receptor co-localizes with proopiomelanocortin expressing neurons in the arcuate nucleus of the hypothalamus. Cultures of neural progenitor cells from the hypothalamus demonstrated erythropoietin, like leptin, stimulated STAT3 activation and proopiomelanocortin production and responses to erythropoietin and leptin were absent in neural progenitor cell cultures that lack erythropoietin receptor expression, and baseline STAT3 activation was reduced and proopiomelanocortin production was decreased in mice with erythropoietin receptor restricted to erythroid tissue, suggesting potential cross talk between erythropoietin and leptin signaling pathways. Erythropoietin stimulated skeletal muscle myoblast proliferation mediated via erythropoietin receptor expression and promoted wound repair in skeletal muscle injury in mice. Erythropoietin contribute to regulation of skeletal myogenic regulatory factors and chronic elevation of erythropoietin in mice increased skeletal muscle myoglobin expression. In vascular endothelium and brain, erythropoietin interacted with nitric oxide synthase activity to regulate oxygen delivery and neuroprotection, respectively. Besides brain, skeletal muscle expressed the highest level of neuronal nitric oxide synthase which contributed to nitric oxide production. We hypothesize that nitric oxide in skeletal muscle can be oxidized by oxy-myoglobin to nitrate and stored as a nitric oxide reservoir in skeletal muscle. Nitrate levels in skeletal muscle and in blood were significantly lower in mice that lack the neuronal isoform of nitric oxide synthase.